Electrophotographic photoreceptor and image forming apparatus having the same

ABSTRACT

An object of the invention is to provide an electrophotographic photoreceptor using a non-contact type charging process excellent in wear resistance life and not causing injury and unevenness in density to the images to be formed for a long time by defining physical properties of the surface. In the electrophotographic photoreceptor using a non-contact type charging process, a creep value C Iτ  is 2.70% or more, preferably 3.00% or more and the Vickers hardness (HV) at the surface is 20 or more and 25 or less in a case where a maximum indenting load of 30 mN is loaded to the surface under a circumstance at a temperature of 25° C. and at a relative humidity of 50%. Since such an electrophotographic photoreceptor ( 1 ) is excellent in flexibility and has plasticity not too soft nor exhibiting fragility, the amount of film reduction due to wear is decreased during long time use, excellent surface smoothness is ensured and there is no occurrence of injury or unevenness in density to the formed images.

TECHNICAL FIELD

The present invention relates to an electrophotographic photoreceptorfor use in electrophotographic image formation, and an image formingapparatus having the same.

BACKGROUND ART

Electrophotographic image forming apparatus have been utilized not onlyfor copying machines but also generally for printers as output means ofcomputers, etc. for which demand has been increased remarkably in recentyears. In the electrophotographic image forming apparatus, toner imagesare formed by uniformly charging a photosensitive layer of anelectrophotographic photoreceptor provided to the apparatus by acharger, exposing the same, for example, by a laser light correspondingto image information, and supplying a particulate developer referred toas a toner from a developing device to electrostatic latent imagesformed by exposure.

While toner images formed by deposition of the toner as an ingredient ofa developer to the surface of the electrophotographic photoreceptor istransferred by transfer means to a transfer material such as recordingpaper. However, not all the toner on the surface of theelectrophotographic photoreceptor is transferred to the recording paperbut the toner partially remains on the surface of theelectrophotographic photoreceptor. Further, paper dusts of recordingpaper in contact with the electrophotographic photoreceptor duringdevelopment may sometimes remain being deposited to theelectrophotographic photoreceptor as they are.

Since the residual toner and the deposited paper dusts on the surface ofthe electrophotographic photoreceptor give undesired effects on thequality of images to be formed, they are removed by a cleaning device.Further, a cleanerless technique has been developed in recent years andthey are removed by a so-called development and cleaning system ofrecovering the residual toner by a cleaning function added to thedeveloping means without providing independent cleaning means. For theelectrophotographic photoreceptor, since operations of charging,exposure, development, transfer, cleaning and charge elimination areconducted repetitively, a durability to electrical and mechanicalexternal forces has been demanded. Specifically, it has been requiredfor durability against abrasion or injury occurred upon frictionalrubbing to the surface of the electrophotographic photoreceptor oragainst degradation of the surface layer caused by deposition of activesubstances such as ozone or NOx generated upon charging by the charger.

For attaining the reduction of cost and free of maintenance in theelectrophotographic image forming apparatus, it is important that theelectrophotographic photoreceptor has sufficient durability and canoperate stably for a long time. The physical property of the surfacelayer constituting the electrophotographic photoreceptor has a greatconcern with the durability and the long time stability of operation ofthe electrophotographic photoreceptor.

Hardness is one of indices for generally evaluating physical propertiesof the materials, particularly, mechanical properties, not beingrestricted only to the physical property on the surface of anelectrophotographic photoreceptor. The hardness is defined as a stressfrom a material against intrusion of an indenter. An attempt ofquantitizing the mechanical property of a film that constitutes thesurface of the electrophotographic photoreceptor by using the hardnessas a physical parameter for recognizing the physical property ofmaterials has been conducted. For example, scratch resistant test,pencil hardness test and Vickers hardness test, etc. have been generallyknown as the test method for measuring the hardness.

However, each of the hardness tests described above involves a problemin measuring the mechanical properties of a material showing complicatebehaviors of plasticity, elasticity (also including retarded component)and creeping property in combination such a film comprising or organicmaterial. For example, while Vickers hardness is used for the evaluationof hardness of a film by measuring the length of an indentation, thisreflects only the plasticity of the film and can not exactly evaluatethe mechanical property such as of those comprising an organic materialshowing a deformation state also including elastic deformation at alarge ratio. Accordingly, the mechanical property of a film constitutedwith an organic material has to be evaluated in view of variousproperties.

One of prior arts for evaluating the physical property of the surfacelayer of the electrophotographic photoreceptor having the organicphotosensitive layer proposes the use of a universal hardness value (Hu)and plastic deformation ratio according to the universal hardness testas specified in DIN 50359-1 (for example, refer to the publication ofJapanese Unexamined Patent Publication JP-A 2000-10320). This prior artdiscloses that mechanical degradation less occurs to the surface layerof the photoreceptor when defining Hu and plastic deformation ratio to aspecified range. However, substantially all light sensitive bodieshaving charge transporting layers using polymeric binders generally usedat present are included in the definition range for the elasticitydisclosed in JP-A 2000-10320 and this results in a problem that asuitable range is not defined substantially.

Further, in another prior art for evaluating the physical property ofthe surface layer of the electrophotographic photoreceptor, it has beendisclosed that the scratch resistance of the photoreceptor can beimproved by defining the Young's modulus as the mechanical propertyother than the hardness to a specified range together with the universalhardness value (Hu) described above in the photoreceptor provided toelectrophotographic image forming apparatus using a contact chargingprocess (for example refer to the publication of Japanese UnexaminedPatent Publication JP-A 2001-125298).

However, another prior art is restricted to the case of using thecontact charging process. In the electrophotographic system using anelectrophotographic photoreceptor for image formation, the process forcharging the photoreceptor is generally classified into two types, i.e.,contact charging as disclosed in another prior art and non-contactcharging using, for example, a scorotron. Accordingly, a difference isnaturally present between the contact charging and non-contact chargingdue to the difference of the charging mode, for the performance requiredfor the photoreceptor used respectively in them. This results in aproblem that a suitable range for defining the surface physical propertyvalue for the electrophotographic photoreceptor using the contact typecharging process can not be applied as it is to the surface physicalproperty of the electrophotographic photoreceptor using the non-contacttype charging process.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide an electrophotographicphotoreceptor using a non-contact type charging process excellent inwear resistance life and not causing injury and unevenness in density tothe images to be formed for a long time by defining physical propertiesof the surface.

The invention provides an electrophotographic photoreceptor in whichelectrostatic latent images are formed by exposure of a surface chargedin a non-contact manner with a light in accordance with imageinformation, toner images are formed by development of the electrostaticlatent images, and obstacles including a toner are removed from thesurface after the toner images are transferred onto a transfer material,wherein

-   -   a creep value C_(Iτ) is 2.70% or more and the Vickers hardness        (HV) at the surface is 20 or more and 25 or less in a case where        a maximum indenting load of 30 mN is loaded to the surface under        a circumstance at a temperature of 25° C. and at a relative        humidity of 50%.

Further, the invention is characterized in that the creep value C_(Iτ)is 3.00% or more.

In accordance with the invention, surface physical properties of anelectrophotographic photoreceptor used for electrophotographic imageformation and charged by a non-contact type charging process are setsuch that the creep value C_(Iτ) is 2.70% or more, preferably, 3.00% ormore in a case where a maximum indenting load of 30 mN is loaded on thesurface under a circumstance at a temperature of 25° C. and at arelative humidity of 50% and a Vickers hardness (HV) at the surface is20 or more and 25 or less. This can maintain the soft and flexibility ofa film forming the surface layer of the electrophotographicphotoreceptor and render the plasticity of the film into a suitablestate which is neither excessively soft nor fragile. Accordingly, evenduring long time use where image formation of charging, exposure,development, transfer, cleaning and charge elimination is repeated,since the amount of film reduction is decreased and occurrence of injuryto the film is decreased to keep the smoothness on the surface of thephotoreceptor, occurrence of injury or unevenness in density to theformed images can be prevented.

Further, the invention provides an image forming apparatus comprising:

an electrophotographic photoreceptor in which the surface is charged ina non-contact manner and a creep value C_(Iτ) is 2.70% or more in a casewhere a maximum intending load of 30 mN is loaded to the surface under acircumstance at a temperature of 25° C. and at a relative humidity of50% and the Vickers hardness (HV) at the surface is 20 or more and 25 orless, charging means for charging the surface of the electrophotographicphotoreceptor in a non-contact manner, exposure means for formingelectrostatic latent images by exposure of the charged surface of theelectrophotographic photoreceptor by a light in accordance with imageinformation, developing means for developing the electrostatic latentimages to form toner images, transfer means for transferring the tonerimages from the surface of the electrophotographic photoreceptor to atransfer material, and cleaning means for cleaning the surface of theelectrophotographic photoreceptor after transfer of the toner images.

Further, the invention is characterized in that the creep value C_(Iτ)in the electrophotographic photoreceptor is 3.00% or more.

In accordance with the invention, since the electrophotographicphotoreceptor excellent in the wear resistance life and scratchresistance is provided, an image forming apparatus not causing injury orunevenness in the density to the formed images is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Other and further objects, features, and advantages of the inventionwill be more explicit from the following detailed description taken withreference to the drawings wherein:

FIG. 1 is a fragmentary cross sectional view schematically showing theconstitution of an electrophotographic photoreceptor 1 according to anembodiment of the invention;

FIG. 2 is a side elevational view for the arrangement schematicallyshowing the constitution of an image forming apparatus 2 according toanother embodiment of the invention having the electrophotographicphotoreceptor 1 shown in FIG. 1;

FIG. 3A and FIG. 3B are charts explaining a method of determining acreep value C_(Iτ);

FIG. 4 is a view showing a relation between Vickers hardness HV andplastic deformation hardness Huplast;

FIG. 5 is a fragmentary cross sectional view schematically showing theconstitution of a photoreceptor 53 as a second embodiment of theinvention; and

FIG. 6 is a view showing a relation between C_(Iτ) and film reductionamount of a photoreceptor.

BEST MODE FOR CARRYING OUT THE INVENTION

Now referring to the drawings, preferred embodiments of the inventionare described below.

FIG. 1 is a fragmentary cross sectional view schematically showing theconstitution of an electrophotographic photoreceptor 1 according to anembodiment of the invention, and FIG. 2 is a side elevational view forthe arrangement schematically showing the constitution of an imageforming apparatus 2 according to another embodiment of the inventionhaving the electrophotographic photoreceptor 1 shown in FIG. 1.

The electrophotographic photoreceptor 1 (hereinafter simply referred toas a photoreceptor) comprises a conductive substrate 3 made of aconductive material, an undercoat layer 4 stacked on the conductivesubstrate 3, a charge generating layer 5 which is a layer stacked on theundercoat layer 4 and containing a charge generating substance, and acharge transporting layer 6 which is a layer stacked further on thecharge generating layer 5 and containing a charge transportingsubstance. The charge generating layer 5 and the charge transportinglayer 6 constitute a photosensitive layer 7.

The conductive substrate 3 has a cylindrical shape and (a) a metalmaterial such as aluminum, stainless steel, copper and nickel, or (b) aninsulating material such as polyester film, phenol resin pipe, or paperpipe provided at the surface thereof with a conductive layer such asaluminum, copper, palladium, tin oxide, or indium oxide is preferablyused. Those having electroconductivity at a volumic resistance of 10¹⁰Ω·cm or less are preferred. The conductive substrate 3 may be appliedwith an oxidation treatment to the surface with an aim of controllingthe volumic resistance. The conductive substrate 3 functions as theelectrode for the photoreceptor 1, as well as also functions as asupport member for each of other layers 4, 5, and 6. The shape of theconductive substrate 3 is not restricted only to the cylindrical shapebut any of plate-like, film-like, or belt-like shape may also be used.

The undercoat layer 4 is formed, for example, of polyamide,polyurethane, cellulose, nitrocellulose, polyvinyl alcohol, polyvinylpyrrolidone, plyacrylamide, anodized aluminum film, gelatin, starch,casein, or N-methoxymethylated nylon. Further, particles such astitanium oxide, tin oxide or aluminum oxide may be dispersed in theundercoat layer 4. The undercoat layer 4 is formed to a thickness ofabout 0.1 to 10 μm. The undercoat layer 4 serves as an adhesive layerbetween the conductive substrate 3 and the photosensitive layer 7, aswell as functions also as a barrier layer that suppresses charges fromflowing to the photosensitive layer 7 from the conductive substrate 3.As described above, since the undercoat layer 4 functions so as tomaintain the charging characteristics of the photoreceptor 1, it canextend the life of the photoreceptor 1.

The charge generating layer 5 can be constituted by incorporation of aknown charge generating substance. As the charge generating substance,any of inorganic pigments, organic pigments and organic dyes can be usedso long as the material absorbs visible rays to generate free charges.The inorganic pigments include selenium and alloys thereof,arsenic-selenium, cadmium sulfide, zinc oxide, amorphous silicon, andother inorganic photoconductors. The organic pigments includephtalocyanine compounds, azo compounds, quinacridone compounds,polycyclic quinone compounds, and perylene compounds. The organic dyesinclude thiapyrylium salts and squarylium salts. Among the chargegenerating substances described above, organic photoconductive compoundssuch as organic pigments and organic dyes are preferably used and, amongthe organic photoconductive compounds, phthalocyanine compounds arepreferably used. Particularly, use of titanylphthalocyanine compounds ismost preferred and satisfactory sensitivity, chargeability andreproducibility can be obtained.

In addition to the pigments and dyes described above, the chargegenerating layer 5 may be incorporated with chemical sensitizers orphotosensitizers. The chemical sensitizer includes electron acceptingmaterials, for example, cyano compounds such as tetracyanoethylene,7,7,8,8-tetracyanoquinodimethane, quinones such as anthraquinone andp-benzoquinone, and nitro compounds such as 2,4,7-trinitrofluolenone and2,4,5,7-tetranitrofluolenone. The photosensitizers include dyes such asxanthene dyes, thiadine dyes, and triphenylmethane dyes.

The charge generating layer 5 is prepared by dispersing the chargegenerating substance described above together with a binder resin in anappropriate solvent, and stacking the same on an undercoat layer 4,followed by drying or curing to form a film. Specifically, the binderresin includes, for example, polyarylate, polyvinyl butyral,polycarbonate, polyester, polystyrene, polyvinyl chloride, phenoxyresin, epoxy resin, silicone, and polyacrylate. The solvent includes,for example, isopropyl alcohol, cyclohexanone, cyclohexane, toluene,xylene, acetone, methyl ethyl ketone, tetrahydrofuran, dioxane,dioxolane, ethylcellosolve, ethyl acetate, methyl acetate,dichloromethane, dichloroethane, monochlorbenzene, and ethylene glycoldimethyl ether.

The solvent is not limited to those described above, but any solventselected from alcohols, ketones, amides, esters, ethers, hydrocarbons,chlorinated hydrocarbons, and aromatics may be used each alone or inadmixture. However, considering the degradation of the sensitivityresulted from crystal relocation upon pulverization and milling of thecharge generating substance and deterioration of characteristics due tothe pot life, use of any one of cyclohexanone, 1,2-dimethoxyethane,methyl ethyl ketone and tetrahydroquione, in organic or inorganicpigments, which cause less crystal relocation are preferred.

For the formation of the charge generating layer 5, a vapor phasedeposition method such as a vacuum vapor deposition method, sputteringmethod or CVD method, coating method, etc. can be used. In a case ofusing the coating method, a coating solution prepared by pulverizing thecharge generating substance by a ball mill, sand grinder, paint shaker,or ultrasonic disperser and dispersing the same in a solvent, andoptionally adding a binder resin is applied on an undercoat layer 4 by aknown coating method. In a case where the conductive substrate 3 formedwith the undercoat layer 4 has a cylindrical shape, a spray method,vertical ring method, or dip coating method can be used as the coatingmethod. The film thickness of the charge generating layer 5 is,preferably, about from 0.05 to 5 μm and, more preferably, about from 0.1to 1 μm.

In a case where the conductive substrate 3 formed with the undercoatlayer 4 is in a sheet-like shape, an applicator, bar coater, casting, orspin coating can be used for the coating method.

The charge transporting layer 6 can be constituted with incorporation ofa known charge transporting substance and a binder resin. It may sufficethat the charge generating layer 6 has an ability of accepting chargesgenerated from the charge generating substance contained in the chargegenerating layer 5 and transferring them. The charge transportingsubstance includes electron donating materials, for example apoly-N-vinylcarbazole and derivative thereof,poly-g-carbazolylethylglutamate and derivative thereof, polyvinylpyrene, polyvinyl phenanthrene, oxazole derivative, an oxadiazolederivative, an imidazole derivative, 9-(p-diethylaminostyryl)anthracene, 1,1-bis(4-dibenzylaminophenyl)propane,styrylanthracene, styrylpyrazoline, pyrazoline derivative,phenylhydrazones hydrazone derivatives, triphenylamino compounds,tetraphenyldiamine compounds, stylbene compounds, or azine compoundssuch as 3-methyl-2-benzothiazoline ring.

The binder resin which constitutes the charge transporting layer 6 maybe those compatible with the charge transporting substance and includes,for example, polycarbonate and copolymerized polycarbonate,polyallylate, polyvinyl butyral, polyamide, polyester, epoxy resin,polyurethane, polyketone, polyvinyl ketone, polystyrene, polyacrylamide,phenol resin, phenoxy resin and polysulfone resin, and copolymer resinsthereof. Those resins can be used each alone or two or more of them maybe used in admixture. Among the binder resins described above, resinssuch as polystyrene, polycarbonate and copolymerized polycarbonate,polyallylate and polyester have a volumic resistivity of 10¹³Ω or moreand have excellent film-forming property, potential characteristics,etc.

As the solvent for dissolving the materials described above, alcoholssuch as methanol and ethanol, ketones such as acetone, methyl ethylketone and cyclohexanone, ethers such as ethyl ether, tetrahydrofuran,dioxane and dioxolane, halogenated aliphatic hydrocarbons such aschloroform, dichloromethane and dichloroethanes and aromatics such asbenzene, chlorobenzene and toluene can be used.

The coating solution for charge transporting layer for forming thecharge transporting layer 6 is prepared by dissolving the chargetransporting substance in a binder resin solution. The ratio of thecharge transporting substance based on the charge transporting layer 6is preferably within a range from 30 to 80% by weight. The formation ofthe charge transporting layer 6 on the charge generating layer 5 isconducted in the same manner as the formation of the charge generatinglayer 5 on the undercoat layer 4. The thickness of the chargetransporting layer 6 is preferably from 10 to 50 μm and, morepreferably, from 15 to 40 μm.

The charge transporting layer 6 may be incorporated with one or moreelectron accepting materials or dyes, for improving the sensitivity andsuppressing the increase of residual potential and fatigue duringrepetitive use. The electron accepting materials include acid anhydridessuch as succinic acid anhydride, maleic acid anhydride, phthalic acidanhydride, 4-chlornaphthalic acid anhydride, cyano compounds such astetracyanoethylene, terephthal malonedinitrile, aldehydes such as4-nitrobenzaldehyde, anthraquinones such as anthraquinone and1-nitroanthraquinone, polycyclic or heterocyclic nitro compounds such as2,4,7-trinitrofluolenone, and 2,4,5,7-tetranitrofluolenone, and they canbe used as a chemical sensitizer.

The dye includes, for example, organic photoconductive compounds such asxanthene dyes, thiadine dyes, triphenylmethane dyes, quinoline pigmentsand copper phthalocyanine. They can be used as the photosensitizer.

Further, the charge transporting layer 6 may be incorporated with aknown plasticizer to improve the moldability, flexibility and mechanicalstrength. The plasticizer includes, for example, dibasic acid ester,fatty acid ester, phosphate ester, phthalate ester, chlorinated paraffinand epoxy type plasticizer. In addition, the photosensitive layer 7 maybe incorporated, for example, with a leveling agent for preventingorange-peel appearance such as polysiloxane, phenolic compounds forimproving durability, an anti-oxidant such as hydroquinone compounds,tocopherol compounds and amine compounds, and UV ray absorbers.

The physical property of the surface film of the photoreceptor 1constituted as described above, that is, the physical property of thesurface film of the photosensitive layer 7 formed into a film shape isset such that a creep value C_(Iτ) is 2.70% or more and, preferably,3.00% or more and a Vickers hardness (HV) at the surface is 20 or moreand 25 or less in a case where a maximum indenting load of 30 mN isloaded on the surface under a circumstance at a temperature of 25° C.and at a relative humidity of 50%.

Now the creep value C_(Iτ) is to be described. Generally, a solidmaterial, even under a relatively low load, gradually develops acontinuous deformation phenomenon, so-called creep, along with lapse oftime retaining applied load. The creep develops remarkably,particularly, in organic polymeric materials. The creep generallyincludes retarded elastic deformation component and plastic deformationcomponent which is used as an index representing the soft andflexibility of the material. FIG. 3A and FIG. 3B are charts forexplaining a method of determining the creep value C_(Iτ) and theVickers hardness HV of a photoreceptor. The creep value C_(Iτ) is aparameter for evaluating the amount of change of the indenting amount ofan indenter under a state of applying a predetermined load for apredetermined time on the surface of a photoreceptor by way of theindenter, that is, the degree of relaxation of the surface film of thephotoreceptor relative to the indentation load.

Hysteresis profiles 8 shown in FIG. 3A and FIG. 3B show the deformation(change of indented depth) hysteresis of an indenting process fromstarting the application of indenting load to the surface of thephotoreceptor 1 till reaching a predetermined maximum indentation loadFmax (A B), a load retaining process for retaining the maximumindentation load Fmax for a predetermined time t (B→C), and a loadremoving process from starting the load removal till reaching a zeroload (0) to complete load removal (C→D). The creep value C_(Iτ) is givenas the amount of change of the indenting amount in the load retainingprocess (B→C).

In this embodiment, the creep value C_(Iτ) was measured by using adiamond indenter (Vickers indenter) of a square pyramidal shape as anindenter under a circumstance at a temperature of 25° C. at a relativehumidity of 50% and under the condition of retaining the load for apredetermined period: t=5 sec at the maximum indentation load: Fmax=30mN. The creep value CIT is specifically given by the following equation(1):C _(Iτ)=100×(h ₂ −h ₁)/h ₁  (1)in which h₁: indented depth at the instance (B) reaching the maximumload 30 mN

h₂: indented depth at the instance (C) after retained for a time t underthe maximum load of 30 mN.

Such creep value C_(Iτ) is determined, for example, by a Fisher ScopeH100V (manufactured by Fisher Instrument Co.)

The reason for defining the creep value C_(Iτ) for the surface of thephotoreceptor 1 is to be described. While the surface of thephotoreceptor 1 is deformed by an energy given when a cleaning member orthe like is indented, the internal energy caused by deformation isrelaxed (dispersed) to suppress proceeding of wear by defining the creepvalue C_(Iτ) to 2.70% or more thereby providing soft and flexibility.That is, the wear resistance life of the photoreceptor is improved. In acase where the creep value C_(Iτ) is less than 2.70%, the soft andflexibility on the surface of the photoreceptor is poor and the wearresistance due to the frictional rubbing with the cleaning member or thelike is lowered to shorten the life.

While the upper limit for the creep value C_(Iτ) is not particularlylimited, it is preferably set to 5.0% or less. In a case where the creepvalue C_(Iτ) exceeds 5.0%, the surface of the photoreceptor becomesexcessively soft and flexible and, the deformation amount by indentationupon frictional rubbing, for example, with a cleaning member is largesometimes failing to obtain a sufficient cleaning effect.

Then, the Vickers hardness HV is to be described. The Vickers hardness(HV) is an index of the plasticity of a material, which is determinedaccording to Japanese Industrial Standard (JIS) Z2244. The Vickershardness (HV) in this embodiment is obtained by determining a plasticdeformation hardness Huplast at first based on an intercept hr where atangential line, relative to a point C of a load elimination curveobtained in the load elimination process (C→D) in the hysteresis profile8 upon determining the creep value C_(Iτ) described above, intersectsthe axis for the indentation depth and a maximum intending load Fmax,and as a value corresponding to the plastic deformation hardnessHuplast. Specifically, the plastic deformation hardness Huplast isobtained by the equation (2).Huplast=Fmax/A(hr)  (2)in which A(hr) is an indentation surface area at the intercept hrdescribed above referred to as a repulsion indented depth and is givenas: A(hr)=26.43·hr².

FIG. 4 is a graph showing a relation between the Vickers hardness HV andthe plastic deformation hardness Huplast. As shown in FIG. 4, sincethere is an extremely high correlation between the Vickers hardness HVand the plastic deformation hardness Huplast, the Vickers hardness HVcorresponding to the plastic deformation hardness Huplast can bedetermined, in other words can be converted. Such Vickers hardness HVcan be determined, for example, by a Fisher Scope H100V in the samemanner as the creep value described above also including the conversionfrom the plastic deformation hardness Huplast to the Vickers hardnessHV.

The reason for defining the Vickers hardness HV of the surface of thephotoreceptor 1 is to be described. In a case where HV is less than 20,the mechanical strength at the surface is insufficient as thephotoreceptor used for the electrophotographic system. On the otherhand, in a case where HV exceeds 25, fragility on the surface of thephotoreceptor develops in which occurrence of injury at the surface ofthe photoreceptor increases and the durability is worsened. Accordingly,the Vickers hardness HV was set to 20 or more and 25 or less.

The photoreceptor 1 in which the creep value C_(Iτ) and the Vickershardness HV are set to a predetermined range, soft and flexibility ofthe film upon forming the surface layer, that is, the photosensitivelayer 7 is maintained and the plasticity of the film is neitherexcessively soft nor fragile. Accordingly, since the amount of filmreduction is decreased and occurrence of injury to the film is alsomitigated to keep the smoothness on the surface of the photoreceptoreven during long time use where image formation of charging, exposure,development, transfer, cleaning, and charge elimination is conductedrepetitively, this can prevent occurrence of injury or unevenness in thedensity in the formed images. The control for the creep value C_(Iτ) andthe Vickers hardness HV on the surface of the photoreceptor 1 isattained by controlling, for example, the kind and the blending ratio ofthe charge transporting substance and the binder resin constituting thephotosensitive layer 7, stacked structure of the photosensitive layer 7,for example, combination of the thickness of the charge generating layer5 and the thickness of the charge transporting layer 6, and the heattreatment condition after forming the charge generating layer 5 and thecharge transporting layer 6.

Then, the operation of forming electrostatic latent images in thephotoreceptor 1 is to be described briefly. The photosensitive layer 7formed to the photoreceptor 1 is uniformly charged, for example,negatively by a charger or the like. When a light having an absorptionwavelength is irradiated to the charge generating layer 5 in the chargedstate, charges of electrons and holes are generated in the chargegenerating layer 5. The holes are transferred by the charge transportingsubstance contained in the charge transporting layer 6 to the surface ofthe photoreceptor 1 to neutralize negative charges on the surface.Electrons in the charge generating layer 5 transfers on the side of theconductive substrate 3 where positive charges are induced to neutralizethe positive charges. As described above, difference is caused betweenthe charged amount in the exposed portion and the charged amount in thenon-exposed portion to form electrostatic latent images to thephotosensitive layer 7.

Then, with reference to FIG. 2, the constitution and the image formingoperation of the image forming apparatus 2 having the photoreceptor 1described above are to be explained. An image forming apparatus 2exemplified in this embodiment is a digital copying machine 2.

The digital copying machine 2 has a constitution generally comprising ascanner station 11 and a laser recording section 12. The scanner station11 includes a document platen 13 comprising transparent glass, areversible automatic document feeder for both surfaces (RADF) 14 forsupplying and feeding documents automatically onto the document platen13 and a scanner unit 15 which is a document image reading unit forscanning images of an original document placed on the document platen 13and reading them. Document images read by the scanner station 11 aresent as image data to an image data input station to be described later,and predetermined image processing is applied to the image data. In theRADF 14, A plurality of documents are set at the same time on a documenttray not illustrated provided to RADF 14. RADF 14 is a device forfeeding the set documents one by one automatically onto the documentplaten 13. Further, RADF 14 comprises a conveying path for documents ofa single surface, a conveying path for documents of both surfaces, meansfor switching the conveying paths, a group of sensors for recognizingand controlling the state of documents passing through each of thestations, a control station, etc so as to cause the scanner unit 15 toread one surface or both surfaces of the document in accordance with theoperator's selection.

The scanner unit 15 comprises a lamp reflector assembly 16 for exposingthe surface of a document, a first scanning unit 18 mounting a firstreflection mirror 17 for reflecting the reflection light from thedocument for introducing the reflection light images from the documentto a photoelectronic conversion device (simply referred to as CCD) 23, asecond scanning unit 21 for mounting second and third reflection mirrors19 and 20 for introducing the reflection light images from the firstreflection mirror 17 to the CCD 23, an optical lens 22 for focusingreflection optical images from the document by way of each of thereflection mirrors 17, 19, and 20 to the CCD 23 that converse them intoelectrical image signals, and the CCD 23.

The scanner station 11 is constituted so as to successively feed andplace the documents to be read on the document platen 13 by theinterlocking operation of the RADF 14 and the scanner unit 15 and readthe document images by moving the scanner unit 15 along the lowersurface of the document platen 13. The first scanning unit 18 scans at aconstant velocity V in the direction of reading the document imagesalong the document platen 13 (from left to right relative to the drawingsheet in FIG. 2). The second scanning unit 21 scans in parallel in theidentical direction at a ½ speed (V/2) relative to the speed V. By theoperation of the first and the second scanning units 18 and 21, imagesof documents placed on the document platen 13 are focused on every linesuccessively to the CCD 23 and images can be read.

The image data obtained by reading the document images in the scannerunit 15 are sent to an image processing station to be described laterand, after being applied with various kinds of image processing, areonce stored in a memory of the image processing station. The image datain the memory are read out in accordance with the output instruction,and the read out image data are transferred to the laser recordingsection 12 to form images on the recording paper as the recordingmedium.

The laser recording section 12 comprises a recording paper conveyingsystem 33, a laser writing unit 26, and an electrophotographicprocessing station 27 for forming images. The laser writing unit 26comprises a semiconductor laser light source for emitting a laser lightin accordance with image data read out from the memory after being readby the scanner unit 15 and stored in the memory, or image datatransferred from an external device, a polygonal mirror for deflectingthe laser light at an equi-angular speed, and an f-θ lens forcompensating the laser light deflected at an equi-angular speed so as tobe deflected at the equi-angular speed on the photoreceptor 1 providedto the electrophotographic processing station 27.

In the electrophotographic processing station 27, a charger 28, adeveloping device 29, a transfer device 30, and a cleaning device 31 arearranged at the periphery of a photoreceptor 1 in this order from theupstream to the down stream in the rotational direction of thephotoreceptor 1 shown by an arrow 32. As described above, thephotoreceptor 1 is uniformly charged by the charger 28 and exposed inthe charged state by the laser light corresponding to the document imagedata emitted from the laser writing unit 26. Electrostatic latent imagesformed by exposure to the surface of the photoreceptor 1 are developedby the toner supplied from the developing device 29 into toner images asvisible images. The toner images formed on the surface of thephotoreceptor 1 are transferred by the transfer device 30 onto recordingpaper as a transfer material fed from a conveying system 33 to bedescribed later.

The photoreceptor 1 rotating further in the direction of the arrow 32after transfer of toner images to the recording paper is frictionallyrubbed at the surface thereof with a cleaning blade 31 a provided to thecleaning device 31. Toner forming the toner images on the surface of thephotoreceptor 1 is not entirely transferred onto the recording paper butsometimes remains slightly on the surface of the photoreceptor 1. Thetoner remaining on the surface of the photoreceptor is referred to asthe residual toner and, since the presence of the residual toner causesdegradation of the quality of the formed images, it is removed andcleaned from the surface of the photoreceptor together with otherobstacles such as paper dusts by the cleaning blade 31 a pressed to thesurface of the photoreceptor.

The conveying system 33 for the recording paper comprises a conveyingsection 34 for conveying recording paper to the electrophotographicprocessing station 27, for conducting image formation, particularly, toa transfer position where the transfer device 30 is located, first tothird cassette feeders 35, 36, and 37 for sending the recording paperinto the conveying section 34, a manual feeder 38 for properly feedingrecording paper of a desired size, a fixing device 39 for fixing images,particularly, toner images transferred from the photoreceptor 1 to therecording paper, and a re-feeding path 40 for re-feeding the recordingpaper for forming images further to the rear face of the recording paperafter fixing of toner images (surface on the side opposite to thesurface formed with the toner images). A plurality of conveying rollers41 are arranged along the conveying paths of the conveying system 33 andthe recording paper is conveyed by the conveying rollers 41 to apredetermined position in the conveying system 33.

The recording paper applied with a fixing treatment for the toner imagesby the fixing device 39 is fed to the re-feeding path 40 for formingimages on the rear face, or fed to a post-processing device 43 by adischarge roller 42. The recording paper fed to the re-feeding path 40is applied with the foregoing operation repetitively and images areformed at the rear face thereof. The recording paper fed to thepost-processing device 43 is applied with post-processing and thendischarged to either first or second discharge cassette 44 or 45 as adesignation of discharge determined depending on the post-processingstep. Thus, a series of image forming operations in the digital copyingmachine 2 is completed.

The photoreceptor 1 provided to the digital copying machine 2 isexcellent in the soft and flexibility of the film that forms thephotosensitive layer 7, and the plasticity of the film is notexcessively soft or it is not fragile. Accordingly, since the amount offilm reduction in the photoreceptor 1 is decreased, and occurrence ofinjury to the film is also decreased to keep the smoothness on thesurface of the photoreceptor 1, an image forming apparatus not sufferingfrom injury or unevenness in the density for images to be formed can beattained.

FIG. 5 is a fragmentary cross sectional view schematically showing theconstitution of a photoreceptor 53 as a second embodiment of theinvention. The photoreceptor 53 in this embodiment is similar with thephotoreceptor 1 of the first embodiment, with corresponding portionscarrying same reference numerals, for which descriptions are to beomitted. What is to be noted in the photoreceptor 53 is that aphotosensitive layer 54 comprising a single layer is formed on aconductive substrate 3.

The photosensitive layer 54 is formed by using the same chargegenerating substance, charge transporting substance, binder resin, etc.as those used for the photoreceptor 1 of the first embodiment. A singlephotosensitive layer is formed on a conductive substrate 3 by the samemethod as that for forming the charge generating layer 5 in thephotoreceptor 1 of the first embodiment, by using a coating solution forphotosensitive layer prepared by dispersing the charge generatingsubstance and the charge transporting substance in the binder resin ordispersing the charge generating substance in the form of pigmentparticles in the photosensitive layer containing the charge transportingsubstance. Since the single layered type photoreceptor 53 of thisembodiment has the photosensitive layer 54 to be coated consisting ofone layer, it is excellent compared with the stacked type constituted bystacking the charge generating layer and the charge transporting layerin view of the production cost and the yield.

EXAMPLE

The invention will be explained with reference to examples.

At first, description is to be made for light sensitive bodies providedas examples and comparative examples by forming photosensitive layersunder various conditions on cylindrical conductive substrates made ofaluminum 30 mm in diameter and 346 mm in length.

Examples 1 to 3

3 parts by weight of titanium oxide TTO-MI-1 (dendritic rutile typetitanium oxide treated at the surface with Al₂O₃ and ZrO₂, titaniumingredient 85%, manufactured by Ishihara Sangyo Co. Ltd.) and 3 parts byweight of an alcohol soluble nylon resin CM 8000 (manufactured by TorayIndustries Inc.) were added to a mixed solvent of 60 parts by weight ofmethyl alcohol and 40 parts by weight of 1,3-dioxolane, which wasdispersed by a paint shaker for 10 hours to prepare a coating solutionfor undercoat layer. The coating solution was filled in a coatingvessel, a conducive substrate was dipped therein and then pulled up, andspontaneously dried to form an undercoat layer having a layer thicknessof 0.9 μm.

10 parts of a butyral resin S-LEC BL-2 (manufactured by Sekisui ChemicalCo. Ltd.), 1400 parts by weight of 1,3-dioxolane, and 15 parts by weightof titanyl phthalocyanine represented by the following structuralformula (1) were put to dispersion treatment by a ball mill for 72hours, to prepare a coating solution for charge generating layer. Thecoating solution was applied on the undercoat layer by the dip coatingmethod in the same manner as in the case of the undercoat layer andspontaneously dried to form a charge generating layer having a layerthickness of 0.4 μm.

Then, as the charge transporting substance, 100 parts by weight of thebutadiene series compound represented by the structural formula (2), 48parts by weight, 32 parts by weight and 32 parts by weight three typesof a polycarbonate resins J-500, G-400, and GH503 (manufactured byIdemitsu Kosan Co., Ltd.), 48 parts by weight of a polycarbonate resinTS2020 (manufactured by Teijin Chemicals Ltd.) and, further, 5 parts byweight of Sumilizer BHT (manufactured by Sumitomo chemical Co., Ltd.)were mixed and dissolved in 980 parts by weight of tetrahydrofuran toprepare a coating solution for charge transporting layer. The coatingsolution was applied on the charge generating layer by a dip coatingmethod, and dried at 130° C. for 1 hour to form a charge transportinglayer having a layer thickness of 28 μm. Thus, a photoreceptor ofExample 1 was prepared.

Example 2

An undercoat layer and a charge generating layer were formed in the samemanner as in Example 1. Then, 100 parts by weight of an enamine seriescompound shown by the following structural formula (3) as the chargetransporting substance, and 99 parts by weight and 81 parts by weight oftwo types of polycarbonate resins GK-700 and GH503 (manufactured byIdemitsu Kosan Co., Ltd.) were dissolved in 1050 parts by weight oftetrahydrofuran to prepare a coating solution for charge transportinglayer. Using the coating solution, a photoreceptor of Example 2 wasprepared in the same manner as in Example 1.

Example 3

A photoreceptor of Example 3 was prepared in the same manner as inExample 2 except for using 99 parts by weight of G-400 (manufactured byIdemitsu Kosan Co., Ltd.) and 81 parts by weight of GH503 (manufacturedby Idemitsu Kosan Co., Ltd.) as the polycarbonate resin upon forming thecharge transporting layer.

Comparative Examples 1 to 5 Comparative Example 1

An undercoat layer and a charge generating layer were formed in the samemanner as in Example 1. Then, 100 parts by weight of a butadiene seriescompound represented by the structural formula (2), 88 parts by weightof a polycarbonate resin G-400 (manufactured by Idemitsu Kosan Co.,Ltd.) and 72 parts by weight of a polycarbonate resin TS2020(manufactured by Teijin Chemicals Ltd.) and, further, 5 parts by weightof Sumilizer-BHT (manufactured by Sumitomo Chemical Co. Ltd.) were mixedas the charge transporting substance and dissolved in 980 parts byweight of tetrahydrofuran to prepare a coating solution for chargetransporting layer. Using the coating solution, a photoreceptor ofComparative Example 1 was prepared in the same manner as in Example 1.

Comparative Example 2

An undercoat layer and a charge generating layer were formed in the samemanner as in Example 1. Then, 100 parts by weight of a enamine compoundrepresented by the structural formula (3), 99 parts by weight of apolycarbonate resin GH-503 (manufactured by Idemitsu Kosan Co., Ltd.),and 81 parts by weight of a polycarbonate resin M-300 (manufactured byIdemitsu Kosan Co., Ltd.) were dissolved as the charge transportingsubstance in 1050 parts by weight of tetrahydrofuran to prepare acoating solution for charge transporting layer. Using the coatingsolution, a photoreceptor of Comparative Example 1 was prepared in thesame manner as in Example 1.

Comparative Example 3

A photoreceptor of Comparative Example 3 was prepared in the same manneras in Comparative Example 2 except for using 180 parts by weight ofM-300 (manufactured by Idemitsu Kosan Co., Ltd.) as the polycarbonateresin upon forming the charge transporting layer.

Comparative Example 4

An undercoat layer and a charge generating layer were formed in the samemanner as in Example 1. Then, 100 parts by weight of a styryl seriescompound represented by the structural formula (4), 105 parts by weightof a polycarbonate resin G-400 (manufactured by Idemitsu Kosan Co.,Ltd.), 45 parts by weight of a polycarbonate resin V290 (manufactured byToyobo Co.) and, further, one part by weight of Sumilizer BHT(manufactured by Sumitomo Chemical Co., Ltd.) were mixed as the chargetransporting substance and dissolved in 980 parts by weight oftetrahydrofuran to prepare a coating solution for charge transportinglayer. Using the coating solution, a photoreceptor of ComparativeExample 4 was prepared in the same manner as in Example 1.

Comparative Example 5

An undercoat layer and a charge generating layer were formed in the samemanner as in Example 1. Then, 100 parts by weight of a butadiene seriescompound represented by the structural formula (2) and 160 parts byweight of a polycarbonate resin G-400 (manufactured by Idemitsu KosanCo., Ltd.) were dissolved as the charge transporting substance in 980parts by weight of tetrahydrofuran to prepare a coating solution forcharge transporting layer. Using the coating solution, a photoreceptorof Comparative Example 5 was prepared in the same manner as in Example1.

As described above, in the preparation for each of the light sensitivebodies of Examples 1 to 3 and Comparative 1 to 5, the creep value C_(Iτ)and the Vickers hardness HV on the surface of the photoreceptor werecontrolled to desired values by changing the type and the content ratioof the charge transporting substance and the resin contained in thecoating solution for charge transporting layer. The creep value C_(Iτ)and the Vickers hardness HV on the surface of the light sensitive bodiesof Examples 1 to 3 and Comparative Examples 1 to 5 were measured by aFisher Scope H100V (manufactured by Fisher Instruments Co.) under thecircumstance at a temperature of 25° C. and at a relative humidity of50%. The measuring conditions included a maximum indentation load: W=30mN, a necessary time of loading up to the maximum indentation load of 10sec, a load retention time: t=5 sec and a load removal time of 10 sec.

Each of the light sensitive bodies of Examples 1 to 3 and ComparativeExamples 1 to 5 was attached to a copying machine AR-450 having anon-contact charging process (manufactured by Sharp Corp.) which wasmodified for testing, and an evaluation test for printing resistance andimage quality stability was conducted by forming images using a genuinetoner for AR-450. Then, the evaluation method for each performance is tobe described.

[Printing Resistance]

The pressure of a cleaning blade of a cleaning device provided to thecopying machine AR-450 abutting against the photoreceptor, a so-called,cleaning blade pressure was adjusted to 21 gf/cm (2.06×10⁻¹ N/cm) as aninitial linear pressure. A character test chart was formed to 100,000sheets of recording paper on every photoreceptor and a printingresistance test was conducted under a normal temperature/normal humidity(N/N) circumstance at a temperature of 25° C. and at a relative humidityof 50%.

The film thickness, that is, the thickness of the photosensitive layerwas measured upon starting the printing resistance test and afterforming images to 100,000 sheets of recording paper by a using aninstantaneous multi-light measuring system MCPD-1100 (manufactured byOhtsuka Electronic Co., Ltd.) by light interference method and the filmreduction amount of the light sensitive drum was determined based on thedifference between the film thickness upon starting the printingresistance test and after forming images for 100,000 sheets of recordingpaper. As the amount of film reduction was larger it was evaluated thatthe printing resistance was worse.

[Image Quality Stability]

After forming images for 100,000 sheets of recording paper in thecopying machine attached with each of the light sensitive bodies,half-tone images were further formed. By visually observing the halftone images with naked eyes, the unevenness in the density of images wasdetected, and the level for the lowering of the image quality of thephotoreceptor, that is, the stability of image quality was evaluatedafter the printing resistance test.

The criterion for the evaluation of unevenness in the density is asdescribed below.

A: good. no unevenness in the density of half-tone images

B: level with no practical problem. slight unevenness in the density ofhalf-tone images

C: level with practical problem. unevenness in the density of half-toneimages

Further, overall judgement for the performance of the photoreceptor wasalso conducted for the amount of film reduction and the unevenness inthe density of half-tone images collectively. The evaluation criterionfor the overall judgement is as described below.

AA: amount of film reduction of less than 1.0 μm and with no unevennessin the density

A: amount of film reduction of 1.0 μm or more and 2.0 μm or less andwith no unevenness in the density

B: amount of film reduction of greater than 2.0 μm or with slightunevenness in the density

C: amount of film reduction of greater than 2.0 μm, and with slightunevenness in the density or with unevenness in the density

The results of evaluation are collectively shown in Table 1. In thephotoreceptor of the examples of the present invention, that is, thephotoreceptor in which the creep value C_(Iτ) was 2.70% or more and theVickers hardness HV was 20 or more and 25 or less, the amount of filmreduction was small and the printing resistance was excellent and nounevenness in the density was observed also in the half-tone imagesafter printing test for 100,000 sheets. Particularly, in the lightsensitive bodies of Examples 2 and 3 with C_(Iτ) of 3.00% or more, theamount of film reduction was extremely small. This is considered toreflect that the photosensitive layers constituting the surface of thelight sensitive bodies of Examples 2 and 3 have soft and flexibility ofthe film represented by the creep property and that the plasticity ofthe film represented by the Vickers hardness HV has a moderate physicalproperty of neither excessively soft nor exhibiting fragility.

On the other hand, while the light sensitive bodies of ComparativeExamples 2 and 3 showed less film reduction amount and excellentprinting resistance since C_(Iτ) was 3.00% or more, unevenness in thedensity of images was observed which is considered to be attributable tothe degradation of the smoothness on the surface of the photoreceptor.This is assumed that the fragility of the film reflecting on the Vickershardness HV was developed. Particularly, in Comparative Example 3, sincethe surface of the photoreceptor was hard, a number of fine scratcheswere formed along the rotational direction on the surface of thephotoreceptor like the surface of an analog record disc by thefrictional rubbing of the photoreceptor by the cleaning blade, anddegradation of the image quality after the printing resistance test wasremarkable.

The light sensitive bodies of Comparative Examples 4 and 5 resulted inextreme increase in the film reduction amount of the photoreceptor. Thisis considered to be attributable to the decrease of the force-moderatingeffect against the press contact force of the cleaning blade to thesurface of the photoreceptor since the creep value C_(Iτ) was small.Further, the smoothness on the surface of the photoreceptor after theprinting resistance test was impaired and degradation of images(unevenness in the density) was confirmed although slightly.

Although the details are not apparent for the reason why the unevennessin the density was formed in the light sensitive bodies of ComparativeExamples 4 and 5, it may be considered as below. That is, in a case ofthe photoreceptor of Comparative Example 4, it is considered that theunevenness in the density was formed because the Vickers hardness HV wasout of the range of the invention in the direction of increasing thehardness, fragility tending to occur in a hard material was developed toresult in uneven wear loss of the film and scatter the exposure laser onthe non-smooth surface of the photoreceptor. Further, also for thephotoreceptor of Comparative Example 5, unevenness in the densityaccompanied by the worsening of the surface smoothness was observed likein Comparative Example 4. Although the cause such as loss of structuraldenseness of the film estimated from the low Vickers hardness HV isconsidered as the factor for worsening the surface smoothness, detailsare not apparent. TABLE 1 Amount Unevenness in of film density Physicalreduction (After printing property value (μm/100k resistance test forOverall C_(Iτ)(%) HV Revolutions) 100,000 sheets) judgement Example 12.88 20.40 1.43 A A Example 2 3.24 23.19 0.45 A AA Example 3 3.10 22.700.82 A AA Comp. 2.68 21.10 2.26 A B Example 1 Comp. 3.35 25.23 0.53 B BExample 2 Comp. 3.49 31.85 0.60 C C Example 3 Comp. 2.16 26.37 2.60 B CExample 4 Comp. 2.13 19.00 2.80 B C Example 5

FIG. 6 is a graph showing a relation between C_(Iτ) and film reductionamount of a photoreceptor. FIG. 6 shows a relation between C_(Iτ) andfilm reduction amount measured for the light sensitive bodies of theexamples and the comparative examples. It can be seen from FIG. 6 thatthe amount of film reduction is decreased apparently as C_(Iτ)increases. Although details are not apparent, it is considered that thesoft and flexibility on the surface of the photoreceptor represented byC_(Iτ) characterizes the film reduction amount, that is, the printingresistance by giving an effect on the degree of moderating the pressingforce by a cleaning blade exerting on the surface of the photoreceptor.

Further, it is considered that the plasticity on the surface of thephotoreceptor represented by the Vickers hardness HV gives an effect onthe smoothness of the surface of the photoreceptor along with printingresistance as described above. Accordingly, it is considered that twofactors of the creep value C_(Iτ) and the Vickers hardness HV have agreat concern as the factor determining the printing resistance and theimage quality stability of the photoreceptor.

As has been described above, while the surface of the photoreceptor isconstituted with a photosensitive layer in this embodiment, this is notrestrictive but it may also be constituted such that a surfaceprotective layer is provided further to the outer layer of thephotosensitive layer and the creep value CIT and the Vickers hardness HVon the surface of the surface protective layer are set to desiredvalues.

INDUSTRIAL APPLICABILITY

In accordance with the invention, the surface physical property of anelectrophotographic photoreceptor used for electrophotographic imageformation and charged by a non-contact type charging process is set suchthat the creep value C_(Iτ) is 2.70% or more, preferably, 3.00% or moreand a Vickers hardness (HV) at the surface is 20 or more and 25 or lessin a case where a maximum indenting load of 30 mN is loaded on thesurface under a circumstance at a temperature of 25° C. and at arelative humidity of 50%. This can maintain the soft and flexibility ofa film forming surface layer of the electrophotographic photoreceptorand render the plasticity of the film into a suitable state which isneither excessively soft nor fragile. Accordingly, even during long timeuse where image formation of charging, exposure, development, transfer,cleaning and charge elimination is repeated, since the amount of filmreduction is decreased and occurrence of injury to the film is decreasedto keep the smoothness on the surface of the photoreceptor, occurrenceof injury or unevenness in the density to the formed images can beprevented.

In accordance with the invention, since the electrophotographicphotoreceptor excellent in the wear resistance life and scratchresistance is provided, an image forming apparatus not causing injury orunevenness in the density to the formed images is obtained.

1. An electrophotographic photoreceptor in which electrostatic latentimages are formed by exposure of a surface charged in a non-contactmanner with a light in accordance with image information, toner imagesare formed by development of the electrostatic latent images, andobstacles including a toner are removed from the surface after the tonerimages are transferred onto a transfer material, wherein a creep valueC_(Iτ) is 2.70% or more and the Vickers hardness (HV) at the surface is20 or more and 25 or less in a case where a maximum indenting load of 30mN is loaded to the surface under a circumstance at a temperature of 25°C. and at a relative humidity of 50%.
 2. The electrophotographicphotoreceptor of claim 1, wherein the creep value C_(Iτ) is 3.00% ormore.
 3. An image forming apparatus comprising: an electrophotographicphotoreceptor in which the surface is charged in a non-contact mannerand a creep value C_(Iτ) is 2.70% or more in a case where a maximumintending load of 30 mN is loaded to the surface under a circumstance ata temperature of 25° C. and at a relative humidity of 50% and theVickers hardness (HV) at the surface is 20 or more and 25 or less,charging means for charging the surface of the electrophotographicphotoreceptor in a non-contact manner, exposure means for formingelectrostatic latent images by exposure of the charged surface of theelectrophotographic photoreceptor by a light in accordance with imageinformation, developing means for developing the electrostatic latentimages to form toner images, transfer means for transferring the tonerimages from the surface of the electrophotographic photoreceptor to atransfer material, and cleaning means for cleaning the surface of theelectrophotographic photoreceptor after transfer of the toner images. 4.The image forming apparatus of claim 3, wherein the creep value C_(Iτ)in the electrophotographic photoreceptor is 3.00% or more.